1. Field of the Technology
The exemplary embodiment described herein relates to a switching apparatus for controlling the interconnection among a plurality of devices. More particularly, the exemplary embodiment relates to a satellite antenna switching apparatus, as installed between a converter that performs a predetermined conversion operation on a signal received by a satellite antenna and a receiver that exchanges signals with the converter, for controlling the interconnection between such a converter and such a receiver on a one-to-many or many-to-many basis.
2. Description of the Prior Art
In reception of satellite broadcast or satellite communication, a high-frequency signal received by a satellite antenna is fed from an LNB (low-noise block converter), which is fitted to the satellite antenna, by way of a coaxial cable or the like to a receiver (for example, a STB (set-top box)). In a case where the received signal outputted from a single LNB is distributed among a plurality of receivers, or in a case where one of the received signals outputted from a plurality of LNBs is selected to be fed to a single receiver, or in a case where the received signals outputted from a plurality of LNBs are distributed among a plurality of receivers in a desired manner, a satellite antenna switching apparatus (hereinafter referred to as a “switch box”) is installed between the LNB(s) and receiver(s) for the purpose of controlling the interconnection between those devices (for example, as disclosed in Japanese Patent Application Laid-Open No. H4-159824).
Some models of conventional switch boxes have, as well as LNB ports to which LNBs are connected and receiver ports to which receivers are connected, a cascade port to which an LNB port of a switch box of the same configuration can be connected. This permits a cascade connection of a plurality of switch boxes.
It is true that a switch box configured as described above permits the number of receivers that share the same LNB to be increased easily simply by connecting another switch box to the cascade port so that the switch boxes are cascade-connected. This makes this type of switch box very useful.
However, with the switch box configured as described above, due to its circuit configuration, when a plurality of them are cascade-connected, the signals exchanged between the receivers and LNBs are attenuated so severely as to make, in the worst case, their detection impossible. Now, how this problem occurs will be explained in detail with reference to FIG. 4. FIG. 4 is a circuit diagram showing the configuration of a principal portion (signal exchange portion) of a conventional switch box. In this figure, for simplicity's sake, it is assumed that only one LNB 31 and one receiver 21 are connected to a first-stage switch box 110, and that only one receiver 22 is connected to a second-stage switch box 120.
When a command signal is fed from the receiver 21 to the LNB 31, according to this command signal, a microcomputer ml turns a transistor n1 on and off so as to vary the current that flows through an impedance circuit z1 that is provided for signal detection. As a result of this control, the potential at a node a1 pulsates according to the command signal, and thus the command signal is, in a form superimposed on a direct-current voltage fed from the receiver 21, fed out via an LNB port LP1. In this way, the LNB 31 is fed with electric power along with the command signal.
On the other hand, when a command signal is fed from the receiver 22 to the LNB 31, according to this command signal, a microcomputer m2 turns a transistor n2 on and off so as to vary the current that flows through an impedance circuit z2. As a result of this current control, the potential at a node a2 pulsates according to the command signal, and thus the command signal is, in a form superimposed on a direct-current voltage fed from the receiver 22, fed out via an LNB port LP2. The voltage fed out via the LNB port LP2 is fed to the cascade port CP1 of the switch box 110, and only the alternating-current component of this voltage is fed through a coupling capacitor c1 to the node a1. Thus, the command signal is, in a form superimposed on the direct-current voltage fed from the receiver 21, fed out via the LNB port LP1.
When these conventional switch boxes 110 and 120 are cascade-connected, however, the impedance circuits z1 and z2, which are both grounded when considered on an alternating-current basis, are connected in parallel with each other (see the equivalent circuits shown in FIGS. 5A and 5B), and thus, assuming that the impedance circuits z1 and z2 are designed to have equal impedances, the actual impedance at the nodes a1 and a2 are reduced to half the impedance as designed. On the other hand, the collector currents of the transistors n1 and n2 are previously determined according to their peripheral constants, and therefore, when the impedance is reduced to a half as described above, the voltage drop at the nodes a1 and a2 (i.e., the amplitude of pulse signals) is accordingly reduced to a half. On the same principle, when n switch boxes are cascade-connected, the amplitude of pulse signals is reduced to 1/n, making it impossible, in the worst case, to detect the exchanged signals as described above.